Method for observing inside of molten iron refining furnace and tuyere for observing inside of furnace

ABSTRACT

It is possible to stably observe the temperature and/or composition of molten iron in a refining furnace by opening a tuyere for observation at all times according to the state of refining. There is provided a method of observing the inside of a molten metal refining furnace comprising the steps of: using a single tube tuyere for observing the temperature and/or composition of molten iron in the refining furnace via a tube penetrating refractories of a furnace wall and/or furnace bottom of the molten iron refining furnace by detecting electromagnetic waves radiated from molten metal at a forward end of the tuyere under a non-contact condition; and using an inert gas or an oxidizing gas, alone or mixed, according to the opening condition of the forward end of the tuyere.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of stably observing thetemperature and/or composition of molten iron, including molten steel,in a refining furnace by detecting electromagnetic waves, which areradiated from molten metal, at an end of a single tube under anon-contact state, via a tube penetrating refractories on a furnace walland/or a furnace bottom of the molten iron refining furnace such as aconverter, an AOD and an RH.

[0003] 2. Description of the Prior Art

[0004] Conventionally, there is provided a method of observing thetemperature and composition of molten iron in a refining furnace, whichis represented by a converter, via a tube penetrating refractories on afurnace wall and/or a furnace bottom.

[0005] For example, the following methods are provided. Concerning thetemperature of molten iron in a refining furnace, there is provided amethod in which an image fiber is used as disclosed in JapaneseUnexamined Patent Publication No. 11-142246. Concerning the compositionof molten iron in a refining furnace, there is provided a method inwhich laser beams are used as disclosed in Japanese Unexamined PatentPublication No. 60-42644.

[0006] In the above techniques, it is necessary for a tuyere used forobservation to be opened at all times. In general, in the case where gasis supplied from a tuyere to molten iron, a piece of solid iron, whichis referred to as a mushroom, is created by coagulation at an end of thetuyere. Due to the creation of this mushroom, it becomes impossible toobserve the temperature and composition of molten iron. In the casewhere the end of the tuyere is opened by an exothermic reaction which iscaused by supplying oxygen gas, the front side of the tuyere is heatedto a high temperature by the heat created in the process of oxidation.Therefore, it becomes impossible to measure the temperature. Further, itbecomes impossible to measure the content of light elements, becausethey are absorbed by oxygen gas. The creation of the mushroom is greatlyaffected by not only the composition and flow rate of the gas suppliedfrom a tube but also the temperature and the components of the moltensteel. However, there is provided no knowledge to make the appropriatecontrol condition clear. As described above, according to the prior art,no knowledge about now to keep open the tuyere used for observation atall times is known. Therefore, it is impossible to stably observe theinside of the furnace in the process of refining.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a method ofstably observing the temperature and/or composition of molten iron,including molten steel, in a refining furnace by keeping a tuyere usedfor observation open at all times, according to the state of refining.It is another object of the present invention to provide a tuyere usedfor observation in the method. The invention will be described asfollows.

[0008] (1) A method of observing the inside of a molten metal refiningfurnace comprising the steps of: using a single tube tuyere forobserving the temperature and/or composition of molten iron in therefining furnace via a tube penetrating refractories of a furnace walland/or furnace bottom of the molten iron refining furnace by detectingelectromagnetic waves radiated from molten metal at a forward end of thetube under a non-contact state; and using an inert gas or an oxidizinggas alone, or mixed with each other, according to the opening conditionof the forward end of the tube. In this case, the inert gas is Ar,nitrogen or CO, and the oxidizing gas is oxygen, air or CO₂.

[0009] (2) A method of observing the inside of a molten metal refiningfurnace according to item (1), wherein a mixed gas of an inert gas withan oxidizing gas or only an oxidizing gas is supplied (the openingperiod) in the case where the ratio (%) of opening of the tube is nothigher than α which is calculated by the inner diameter r (mm) of thetuyere according to Equation (1), and only inert gas is supplied (thesteady period) in the case where the ratio of opening is higher than α.

α=765/r ²  (1)

[0010] In this case, the opening period is judged and completed when thetemperature of molten iron at the forward end of the tube to be measuredis not lower than 1800° C. Although the upper limit of the rate ofopening is not particularly prescribed, it is preferable that the upperlimit of the rate of opening is not more than 95% so as to prevent thefusion of the tuyere.

[0011] (3) A method of observing the inside of a molten metal refiningfurnace comprising the steps of: using a single tube tuyere forobserving the temperature and/or composition of molten iron in therefining furnace via a tube penetrating refractories of a furnace walland/or furnace bottom of the molten iron refining furnace, by detectingelectromagnetic waves radiated from molten metal, at a forward end ofthe tube under a non-contact state; and controlling a flow rate of aninert gas according to the opening condition of the forward end of thetube. In this case, the inert gas is Ar, nitrogen or CO.

[0012] (4) A method of observing the inside of a molten metal refiningfurnace according to item (3), wherein the flow rate of an inert gas iscontrolled according to the temperature and composition of the molteniron so that the ratio (%) of opening of the single tube can be not lessthan α, which is calculated by the inner diameter r (mm) of the tubeaccording to Equation (1), and not more than 95%.

[0013] (5) A tuyere for observing the inside of a molten metal refiningfurnace having a single tube for observing the temperature and/orcomposition of molten iron in the refining furnace, via a tubepenetrating refractories of a furnace wall and/or furnace bottom of themolten iron refining furnace, by detecting electromagnetic wavesradiated from molten metal at a forward end of the tube under anon-contact state, the tuyere for observing the inside of a molten metalrefining furnace comprising a control function by which an inert gas oran oxidizing gas can be used alone, or mixed with each other, accordingto the state of the opening of the forward end of the tube, the innerdiameter of which is 2 to 6 mm.

[0014] (6) A method of observing the inside of a molten metal refiningfurnace in which the temperature and/or composition of the molten ironin the molten iron refining furnace is observed via a tube penetratingrefractories of a furnace wall and/or furnace bottom of the molten metalrefining furnace, in a non-contact state, by detecting electromagneticwaves radiated from molten iron at the forward end of the tube, themethod of observing the inside of the molten metal refining furnacecomprising the steps of: using a twin tube tuyere; detecting a ratio ofopening of the forward end of the inner tube tuyere; and controlling asize of a mushroom at the forward end tuyere by changing gas flow rateand/or gas composition which is supplied through the inner and the outertube according to a change in the ratio of opening so as to keep theratio of opening necessary for observation.

[0015] (7) A method of observing the inside of a molten metal refiningfurnace according to item (6), further comprising the steps of:estimating the size of the mushroom at the forward end of tuyereaccording to the temperature and composition of molten iron; andcontrolling the size of the mushroom at the forward end of tuyere bychanging the gas flow rate of and/or gas composition of LPG inert gas,inert gas and oxidizing gas which are supplied through the outer tubeaccording to the result of the estimation so as to keep the ratio (%) ofopening of the inner tube in a range from not less than α(%), which iscalculated by Equation (5), to not more than 95%.

α=850/r ²  (5)

[0016] where r is an inner diameter (mm) of the inner tube.

[0017] (8) A method of observing the inside of a molten metal refiningfurnace according to item (6), further comprising the steps of:supplying a mixed gas, in which an inert gas and an oxidizing gas aremixed with each other, or only an oxidizing gas from the inner tube, soas to increase the ratio of opening in a tube opening period in the casewhere the ratio of opening of the inner tube is lower than α (%) inEquation (5); and supplying only inert gas from the inner tube in aperiod except for the tube opening period.

[0018] (9) A method of observing the inside of a molten metal refiningfurnace according to one of items (6) to (8), further comprising thesteps of: supplying an inert gas from the inner tube at all times;supplying a mixed gas, in which inert gas and oxidizing gas are mixedwith each other, or only an oxidizing gas, from the outer tube so as toincrease the ratio of opening of the inner tube in a tube opening periodin the case where the ratio of opening of the inner tube is lower than α(%) in Equation (5); and supplying tuyere cooling gas, or inert gasalone, through the outer tube or supplying mixed gas, in which tuyerecooling gas and inert gas are mixed, from the outer tube in a periodexcept for the tube opening period.

[0019] (10) A tuyere for observing the inside of a molten metal refiningfurnace, which is a double tube tuyere for observing the temperatureand/or composition of molten iron in the refining furnace via a tubepenetrating refractories of a furnace wall and/or a furnace bottom ofthe molten iron refining furnace by detecting electromagnetic wavesradiated from molten metal at a forward end of the tube under anon-contact state, the tuyere for observing the inside of a molten metalrefining furnace comprising: a piping structure; and a control systemcapable of independently controlling a gas flow rate and/or gascomposition which is supplied through each of the inner and the outertube.

[0020] (11) A tuyere for observing the inside of a molten metal refiningfurnace according to claim 10, wherein inner diameter r of the innertube is 5 to 20 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a view showing a relation between the diameter (K) of anopening section of a tuyere for observation, the diameter (M) of amushroom created at a forward end of the tuyere, and inner diameter (r)of the inner tube.

[0022]FIG. 2 is a view showing a result of an experiment which shows arelation between parameter α, inner diameter r of a tube and theaccuracy of measurement of temperature by radiation.

[0023]FIG. 3 is a view showing a relation between a ratio of an openingand the accuracy of measurement of temperature by radiation in the casewhere a tube, the inner diameter of which is 10 mm, is used.

[0024]FIG. 4 is a view showing a model of a single tube for observationof the present invention.

[0025]FIG. 5 is a view showing a model of a twin tube for observing theinside of a furnace of the present invention.

DESCRIPTION OF THE MOST PREFERRED EMBODIMENT

[0026] The present invention has been accomplished according to the newknowledge that the opening area of a tuyere for observation and the sizeof a mushroom created at a forward end of the tube are correlated toeach other and the opening area can be controlled when the mushroom sizeis controlled. FIG. 1 is a graph showing a detailed result of anexperiment made by the present inventors in which a melting furnace, thecapacity of which was 1 ton, was used. As shown in FIG. 1, M/r and K/rare strongly correlated to each other wherein r is an inner diameter ofan inner tube of a tuyere, K is a diameter of an opening section of thetuyere for observation, and M is a diameter of a mushroom created at aforward end of the tube. That is, in order to control the ratio ofopening of the tube to be a value necessary for observation, it isnecessary to control the mushroom size by changing the gas flow rate andgas composition.

[0027] In this case, the electromagnetic wave is a generic name foremitted energy such as light used for radiant measurement and light usedfor laser beam emission analysis, the wave-length of which is peculiarto each component. The reason why the single tube is adopted in thepresent invention is that gas is supplied to the single tube from asingle gas generation system, so that the equipment investment is small.The reason why the twin tube tuyere is adopted in the present inventionis that the composition and gas flow rate can be independentlycontrolled by the twin tube tuyere. Gas used for the inner and outertube is a tuyere cooling gas, such as LPG inert gas, an inert gas and anoxidizing gas which are used alone or mixed. Concerning the tuyerecooling gas of the outer tube, by which the cooling effect can bepositively provided when the gas is decomposed. Concerning the inert gasof the outer tube, Ar, nitrogen or carbon monoxide gas is used.Concerning the oxidizing gas of the outer tube, oxygen, air or carbondioxide is used. Concerning the inert gas used for the inner tube, Ar,nitrogen or carbon monoxide gas is used. Concerning the oxidizing gasused for the inner tube, oxygen, air or carbon dioxide is used.

[0028] The first item of the present invention is a method of observingthe inside of a molten metal refining furnace represented by aconverter, an electric furnace or an AOD comprising the steps of: usinga single tube for observing the temperature and/or composition of molteniron in the refining furnace via a tube penetrating refractories of afurnace wall and/or furnace bottom of the molten iron refining furnaceby detecting electromagnetic waves radiated from molten metal at aforward end of the tube under a non-contact state; and using inert gasor oxidizing gas alone or mixed according to the opening condition ofthe forward end of the tube.

[0029] In the single tube, according to the state of opening at aforward end of the tube, inert gas and oxidizing gas are used alone ormixed. That is, observation is conducted by detecting an electromagneticwave radiated from an interface formed between a molten iron face at theforward end of the tube and bubbles of gas blown out from the tuyere.The ratio of opening of the forward end of the tube is controlledaccording to the composition of gas so that an intensity ofelectromagnetic waves can be controlled to be a sufficiently high valueprescribed according to the method of observation. In this case, theinert gas is Ar, nitrogen or CO. The oxidizing gas is oxygen, air orCO₂. In the case where the ratio of opening at the forward end of thetube is too low, the accuracy of observation is deteriorated. Therefore,oxidizing gas is mixed with inert gas, so that the mushroom created atthe forward end of the tube is melted. On the contrary, in the casewhere the ratio of opening at the forward end of the tube is too high,the fusion of the tuyere is great, so that inert gas is used alone andthe mushroom is created as long as the accuracy of observation is notdeteriorated.

[0030] The second item of the present invention prescribes a specificcontrol method in the first invention. In this case, the area of openingnecessary for observation of temperature in which an intensity ofelectromagnetic waves is high is different from the area of openingnecessary for observation in the case of laser emitted light forcomposition analysis in which an intensity of light is low. Further, thearea of opening necessary for observation is different according to theinner diameter and length of the tuyere. In general, when considerationis given to the thickness of refractories of a large-scale converter,the length of the tuyere is approximately 1 to 2 m. In this case, thearea of observation of 6 mm² is required, which is experimentally known.This knowledge is organized into Equation (1). Accordingly, the seconditem of the present invention provides a method of observing the insideof a molten metal refining furnace, wherein a mixed gas of an inert gaswith an oxidizing gas, or only an oxidizing gas, is supplied (in theopening period) in the case where the ratio (%) of opening of the tubeis not higher than α which is calculated by the inner diameter r of thetube according to Formula (1), and only inert gas is supplied (in thesteady period) in the case where the ratio of opening is higher than α.

α=765/r ²  (1)

[0031] In this case, the ratio of opening is defined as a value obtainedwhen an area of the opening, at the forward end of the tube not coveredwith the mushroom, is divided by a cross-sectional area of the tube,wherein this ratio of opening is expressed by percent. In the case wherea relation between the ratio of opening and the back pressure ispreviously measured, it is possible to detect the ratio of opening by achange in the back pressure of gas. Further, it is possible to directlydetect the ratio of opening by the observation conducted by an imagefiber arranged at the forward end of the tuyere on the shell side.

[0032]FIG. 2 is a graph showing an example in which the presentinvention is applied to the measurement of radiation in which an imagefiber is used. The accuracy of the vertical axis corresponds to 2σ(σ isa standard deviation) of the measured temperature. Due to the foregoing,it is understood that the temperature can be observed with accuracy whenα=r² is not lower than 765. However, when α=r² is lower than 765, thevisual field is decreased because of a block in the opening.Accordingly, the accuracy of observation is deteriorated.

[0033] Specifically, control is conducted as follows. In the case wherethe ratio of opening of the tube is lower than the critical valuenecessary for observation, the value of α=r² of which is 765, one of ortwo and more of the rate of flow of oxidizing gas of oxygen, air and CO₂and the flow rate of inert gas of Ar, nitrogen and CO are adjustedaccording to the inner diameter of the tube, the temperature of themolten iron and the concentration of carbon in the molten iron, so thatthe rate of opening can be controlled.

[0034] The diameter of the mushroom formed at the forward end of thetube, which is an index of control, can be calculated according to theheat balance of each item described below. When a relation between thediameter of the mushroom and the rate of opening is experimentallyfound, it is possible to conduct control.

[0035] (1) Index (v1) of cooling by sensible heat of gas: function ofspecific heat of gas

[0036] (2) Index (v2) of cooling by latent heat of gas: function ofreaction heat of gas

[0037] (3) Index (k) of receiving heat of the mushroom from molten iron

[0038] When the mushroom is assumed to be a hemisphere, the followingheat balance is established.

k=M ²×(T−Ts)×Q ^(n) =a+b×(v1+v2)  (2)

[0039] In the above Equation (2), a, b and n are constants, Q is a flowrate of total gas (Nm³/h/t), T is a temperature (° C.) of molten iron,and Ts is a solid line temperature (° C.). In this case, v1 and v2 canbe calculated when a ratio of contribution to the creation of themushroom is determined by an experiment according to the physicalproperty and reaction heat of the used gas. Ts can be found by the phasediagram. When these are put into Equation (2) and the constants aredetermined so that they can agree with the mushroom diameter obtained byan experiment, it is possible to obtain a formula of estimation of themushroom diameter when an actual device is used. In this connection,concerning the ratio of contribution of reaction heat, the presentinventors found the following by an experiment. In the case of oxygen(including a component of oxygen in air), the ratio of contribution is70 to 80% of the latent heat of forming reaction of FeO calculated bythe reaction of 2Fe+O₂=FeO. However, in the case of CO₂, the ratio ofcontribution is only 2 to 5% of the latent heat calculated by thereaction of CO₂+[C]=2CO. Further, according to the experiment made bythe present inventors, diameter M (mm) of the mushroom, the innerdiameter r (mm) of the tube and the diameter K (mm) of the openingsection equivalent to a circle have a relation expressed by thefollowing Equation (3).

(K/r)=β−0.165×(M/r)  (3)

[0040] In the above equation, β is in a range from 1.0 to 1.3.

[0041] The third item of the present invention is a method of observingthe inside of a molten metal refining furnace comprising the steps of:using a single tube for observing the temperature and/or composition ofmolten iron in the refining furnace via a tube penetrating refractoriesof a furnace wall and/or furnace bottom of the molten iron refiningfurnace by detecting electromagnetic waves radiated from molten metal ata forward end of the tube under a non-contact state; and controlling aflow rate of inert gas according to the opening condition of the forwardend of the tube. According to the method, the mushroom size iscontrolled when the flow rate of inert gas is controlled. When the ratioof opening at the forward end of the tube is too low, the accuracy ofobservation is deteriorated. Therefore, when the flow rate of inert gasis decreased so as to decrease the cooling capacity given by thesensible heat of gas, the mushroom created at the forward end of thetube is melted. On the contrary, when the ratio of opening at theforward end of the tube is too high, the tube is greatly melted.Therefore, when the flow rate of inert gas is increased so as toincrease the cooling capacity given by the sensible heat of gas, themushroom is created as long as the accuracy of observation is notdeteriorated.

[0042] This is necessary when the inside of the tube must be kept in anatmosphere of inert gas at all times so that the emitted light can betransmitted without causing attenuation in the case where light of ashort wave-length, which has been emitted from carbon or phosphorus bylaser beams, is observed. The present inventors found that the ratio ofopening can be controlled even if the inside of the tube is kept in anatmosphere of inert gas at all times.

[0043] The fourth and the fifth item of the present invention prescribea specific control method of the third invention. The fourth item of thepresent invention provides a method of observing the inside of a moltenmetal refining furnace, in which a flow rate of inert gas is controlledaccording to the temperature and composition of molten iron so that theratio (%) of opening of the single tube can be not less than α, which iscalculated by the inner diameter r (mm) of the tube according toEquation (1), and not more than 95%. When the ratio of opening is higherthan 95%, the size of the mushroom created at the forward end of thetube is too small. Therefore, it is impossible to protect the tube, andthe life of the tube is short.

[0044] In the fifth item of the present invention, the carbonconcentration can be estimated by a method in which the carbonconcentration is calculated from the quantity of oxygen to be suppliedand the decarbonizing efficiency, which is experimentally known, on thebasis of the carbon concentration of molten iron to be charged. Further,the carbon concentration can be estimated by a method in which thecarbon concentration is estimated from the exhaust gas analysis and theresult of direct sampling of molten iron. Alternatively, the carbonconcentration can be estimated by the combination of the above methods.The temperature can be estimated by a direct continuous measurementmethod or a semi-continuous measurement method. Further, the temperaturecan be estimated by a method in which the temperature is calculated fromthe temperature rising efficiency which is experimentally known.Alternatively, the temperature can be estimated by the combination ofthe above methods. The reason why the flow rate of inert gas iscontrolled according to the temperature and composition of molten ironis that the size of the mushroom is greatly affected by a differencebetween the temperature of molten iron and the temperature of the solidphase line of molten iron. Also, the reason why the flow rate of inertgas is controlled according to the temperature and composition of molteniron is that it is necessary to detect a difference between thetemperature of molten iron and the temperature of the solid phase linewhich is determined by the molten iron composition (the carbonconcentration) and also it is necessary to increase and decrease theflow rate according to the value of difference.

[0045] Also, in this case, the diameter of the mushroom at the forwardend of the tube, which is an index of control, can be calculated by theheat balance of each item. It is possible to control when anexperimental relation between the diameter of the mushroom and the ratioof opening is found.

[0046] (1) Index (v1) of cooling by sensible heat of gas: function ofspecific heat of gas

[0047] (2) Index (k) of receiving heat of the mushroom from molten iron

[0048] When the mushroom is assumed to be a hemisphere, the followingheat balance is established.

k=M ²×(T−Ts)×Q ^(n) =a+b×v1  (4)

[0049] In the above Equation (4), a, b and n are constants, Q is a flowrate of total gas (Nm³/h/t), T is a temperature (° C.) of molten iron,and Ts is a solid line temperature (° C.). In this case, v1 can becalculated according to the physical property of the used gas. Ts can befound according to the phase diagram. When these are put into Equation(4) the constants are determined so that they can agree with themushroom diameter obtained by an experiment. In this way, it is possibleto obtain a formula of estimation of the mushroom diameter when anactual device is used. The relation between diameter M of the mushroomand diameter K of the equivalent circle of the opening section can becalculated by Equation (3).

[0050] Concerning the tuyere, as shown by an embodiment in FIG. 4, thepresent invention provides a tuyere for observing the inside of a moltenmetal refining furnace which is a single tube for observing thetemperature and/or composition of molten iron in the refining furnacevia a tube penetrating refractories of a furnace wall and/or furnacebottom of the molten iron refining furnace by detecting electromagneticwaves radiated from molten metal at a forward end of the tube under anon-contact state, the tuyere for observing the inside of a molten metalrefining furnace comprising a control function by which inert gas oroxidizing gas can be used alone or mixed according to a state of openingof the forward end of the tube.

[0051] In this case, concerning the inner diameter of the tube forobservation, the inner diameter of the tube pipe is 2 to 6 mm. In thecase where the inner diameter of the tube is smaller than 2 mm, it isimpossible to create a mushroom when the opening area necessary forobservation is ensured. Therefore, the life of the tuyere is short. Inthe case where the inner diameter of the tube is larger than 6 mm, theflow rate of gas is increased and the cost is raised, which is noteconomical.

[0052] Next, explanations will be made into a case in which the twintube tuyere of the present invention is adopted.

[0053] According to the sixth item of the present invention,electromagnetic waves are detected which are emitted from an interfaceformed between the molten iron surface at the forward end of the tubeand the bubbles of gas which has been blown into the tuyere. In thiscase, it is necessary to control the ratio of opening at the forward endof the inner tube by the composition and flow rate of gas in the innerand the outer tube so that an intensity of the electromagnetic wavesemitted from the interface can be sufficiently high for the intensityrequired by the method of observation. Therefore, the ratio of openingis detected by the change in the back pressure of gas and the result ofobservation conducted by an image fiber. According to the thus detectedratio of opening, the flow rate and/or composition of gas in the innerand the outer tube is changed so as to change the size of the mushroom.In this way, the ratio of opening necessary for observation is kept.

[0054] The seventh item of the present invention is a specific controlmethod of the above item 6 of the present invention. According to theseventh item of the present invention, when the cooling capacity of theouter tube is controlled according to the temperature and composition ofmolten iron, the ratio of opening of the tuyere is kept to be a valuehigher than the critical value necessary for observation at all times.When the gas flow rate and/or gas composition which are supplied througheach of the tuyere, inert gas and oxidizing gas of the outer tube ischanged according to the mushroom size which is estimated according tothe temperature and composition of molten iron, the ratio (%) of openingof the tube is kept in a range not less than α (%) and not more than95%.

α=850/r ²  (5)

[0055] where r is an inner diameter (mm) of the inner tube. Since it ispreferable that r is not less than 3 mm, α is set at a value lower than95%. Further, it is preferable that inert gas is supplied into the innertube at all times. In this case, the ratio of opening is defined as avalue (%) which is obtained when an area of the opening region at theforward end of the tube not covered with the mushroom is divided by across-sectional area of the tuyere.

[0056] The critical value of the ratio of opening, in the case where theintensity of electromagnetic waves is high such as a case in which thetemperature is observed, is different from the critical value of theratio of opening, in the case where the intensity of electromagneticwaves is low such as a case of light emitted by a laser used forcomponent analysis. Further, the critical value of the ratio of openingis different according to the inner diameter and the length of the tube.In general, when consideration is given to the thickness of refractoriesof a large-scale converter, the length of the tube is approximately 1 to2 m. In this case, the area for observation not less than 6 mm² isrequired, which is experimentally known. This knowledge is organizedinto Equation (5). In the tube, the inner diameter of which is r (mm),in order to provide an area R mm² for observation at the forward end ofthe tube, the ratio of opening must be a value not less than αcalculated by Equation (6).

α=R/(π×(r/2)²)×100=127×R/r ²(%)  (6)

[0057] In this case, when a value not less than 6 mm² is substitutedinto R, Equation (5) can be obtained. In the case where the ratio ofopening is lower than α, the accuracy of observation is deterioratedbecause the area of the opening at the forward end of the tube is small.In the case where the ratio of opening is higher than 95%, since thesize of the mushroom created at the forward end of the tube is toosmall, it is impossible to protect the tuyere. Therefore, the life ofthe tuyere is short. FIG. 3 is a view showing an example of the accuracyof measurement of temperature by radiation in which an image fiber, theinner diameter of which was 10 mm, was used. The accuracy of thevertical axis corresponds to 2σ(σ is a standard deviation) of themeasured temperature. Due to the foregoing, it can be understood thatthe temperature can be accurately observed when the ratio of opening isnot less than 8.5% (corresponding to α in Equation (5)). However, whenthe ratio of opening is lower than 8.5%, the visual field is decreaseddue to blocking by the tuyere. Therefore, the accuracy of observation isdeteriorated. On the contrary, when the ratio of opening is higher than95%, the ratio of opening is so high that the mushroom cannot besufficiently created, and the tuyere is damaged by fusion.

[0058] The present invention has been accomplished according to the newknowledge that the size of the mushroom created at the forward end ofthe tube, which is closely related to the opening area of the tuyere forobservation, is more affected by the outer tube gas than by the innertube gas. Accordingly, in order to control the ratio of opening of thetube, the flow rate and/or composition of the outer tube is controlled.An example of the tuyere cooling gas of the outer tube is LPG. Examplesof the inner gas are Ar, nitrogen and carbon monoxide gas. Examples ofoxidizing gas are oxygen, air and carbon dioxide gas. Specifically, inorder to make the ratio of opening to be higher than α, one of thefollowing actions (1) to (4) is executed so as to raise the temperatureof the forward end of the outer tube of the tuyere, so that the mushroomis melted. In this case, the inner tube is filled with inert gas at alltimes. Therefore, no problems are caused in the measurement ofelectromagnetic waves.

[0059] (1) The flow rate of inert gas is decreased.

[0060] (2) oxidizing gas is mixed with inert gas.

[0061] (3) In the mixed gas in which inert gas and oxidizing gas aremixed with each other, while the total flow rate is being kept constant,the mixing ratio of oxidizing gas is increased, or while the flow rateof inert gas is being kept constant, the flow rate of oxidizing gas isincreased.

[0062] (4) Only oxidizing gas is blown into the tuyere.

[0063] On the contrary, when the ratio of opening is made to be not morethan 95%, at least one of the following actions (1) to (3) is executedso as to decrease the temperature at the forward end of the outer tubeof the tuyere, so that the mushroom is created and the tuyere isprotected. In this case, the inner tube is filled with inert gas at alltimes. Therefore, no problems are caused in the measurement ofelectromagnetic waves.

[0064] (1) The flow rate of inert gas is increased.

[0065] (2) Tuyere cooling gas is mixed with inert gas.

[0066] (3) In the mixed gas in which inert gas and tuyere cooling gasare mixed with each other, while the total flow rate is being keptconstant, the mixing ratio of tuyere cooling gas is increased, or whilethe flow rate of inert gas is being kept constant, the flow rate oftuyere cooling gas is increased.

[0067] Since the behavior of creation of the mushroom is greatlyaffected by the composition and temperature of molten iron, it isnecessary to control according to the composition and temperature ofmolten iron. It is most reasonable that the results of measurement ofthe composition and temperature of molten iron, which have been measuredaccording to the electromagnetic waves obtained through the tuyere forobservation, are used. However, it is possible to estimate the carbonconcentration by the method in which the carbon concentration iscalculated from the quantity of oxygen to be supplied and thedecarbonizing oxygen efficiency, which is experimentally known, on thebasis of the carbon concentration of charged molten iron. Also, it ispossible to estimate the carbon concentration by the method in which thecarbon concentration is estimated from the results of exhaust gasanalysis or direct sampling of molten iron. The carbon concentration canbe estimated by one of the above methods or the combination of the abovemethods. Further, the temperature can be estimated by the method inwhich the temperature is calculated from the temperature risingefficiency, which is experimentally known, on the basis of thetemperature of the charged molten iron.

[0068] Specifically, according to the relation shown in FIG. 1, diameterM of the mushroom created at the forward end of the tube is controlledas M/r. Diameter M of the mushroom can be estimated in such a mannerthat diameter M of the mushroom is calculated by the heat balance ofeach of the following items (1) to (4).

[0069] (1) Cooling index (v1) by sensible heat of outer tube gas:function of specific heat of outer tube gas

[0070] (2) Cooling index (v2) by latent heat of outer tube gas: functionof reaction heat of outer tube gas

[0071] (3) Cooling index (v3) by sensible heat of inner tube gas:function of specific heat of inner tube gas

[0072] (4) Heat receiving index (k) from molten iron of mushroom

[0073] When the mushroom is assumed to be a hemisphere, the followingheat balance is established.

k=M ²×(T−Ts)×Q ^(n) =a+b×(v1+v2+v3)  (7)

[0074] In the above Equation (7), a, b and n are constants, Q is a flowrate of total gas (Nm³/h/t), T is a temperature (° C.) of molten iron,and Ts is a solid line temperature (° C.) determined by the compositionof the molten iron. In this case, v1, v2 and v3 can be calculated when aratio of contribution to the creation of the mushroom is determined byan experiment according to the physical property and reaction heat ofthe used gas. Ts can be found according to the phase diagram. When theseare put into Equation (7) and the constants are determined so that theycan agree with the mushroom diameter obtained by an experiment. In thisway, it is possible to obtain a formula for estimating the mushroomdiameter when an actual device is used.

[0075] The eighth item of the present invention provides a method ofopening a tuyere by supplying oxidizing gas from the inner tube when thetuyere is blocked. That is, the present invention provides a method ofobserving the inside of a molten metal refining furnace, furthercomprising the steps of: supplying a mixed gas, in which an inert gasand an oxidizing gas are mixed, or containing only an oxidizing gas,from the inner tube so as to increase the ratio of opening in a tuyereopening period in the case where the ratio of opening of the tube islower than α (%) in Equation (5); and supplying only inert gas from theinner tube in a period except for the tuyere opening period. In thiscase, the tuyere opening period is defined as a period from the point intime at which the ratio of opening becomes lower than α so that theaction to open the opening is executed to the point in time at which theratio of opening becomes a value not less than 95%. According to theknowledge of the inventors, in the case where the ratio of openingcannot be measured because the temperature of the forward end of thetube is high, it can be judged that the tuyere has been opened when thetemperature of the forward end of the tube is raised to a temperaturenot lower than 1800° C., and the tuyere opening period can be ended.Concerning the action to open the opening, one of the following actions(1) and (2) or both of the following actions (1) and (2) may beexecuted, so that the temperature of the forward end of the tube israised so as to melt the mushroom.

[0076] (1) The inner tube is filled with mixed gas in which inert gasand oxidizing gas are mixed with each other. While the total flow rateis being kept constant, the mixing ratio of oxidizing gas is increased.Alternatively, while the flow rate of the inert gas is being keptconstant, the flow rate of oxidizing gas is increased.

[0077] (2) Only oxidizing gas is blown from the inner tube.

[0078] In this case, the reason why the action to open the opening isconducted in the inner tube is that it is possible to increase the flowrate of gas so that opening can be positively conducted in a shortperiod of time. Specifically, as can be seen in FIG. 1, when M/r is notmore than 2, K/r becomes not less than 1. The fact that K/r is 1 meansthat the opening diameter and the tuyere diameter coincide with eachother. That is, the tuyere is completely open. Accordingly, in the casewhere the tuyere is blocked, the action is taken by which M/r becomesnot more than 2, so that the tuyere can be opened since K/r is made tobe not less than 1. Estimation of diameter M of the mushroom can becalculated by the heat balance described in each of the following items.

[0079] (1) Cooling index (v1′) by sensible heat of outer tube gas:function of specific heat of outer tube gas

[0080] (2) Cooling index (v2′) by latent heat of outer tube gas:function of reaction heat of outer tube gas

[0081] (3) Cooling index (v3′) by sensible heat of inner tube gas:function of specific heat of inner tube gas

[0082] (4) Cooling index (v4′) by latent heat of inner tube gas:function of reaction heat of inner tube gas

[0083] (5) Heat receiving index (k′) from molten iron of mushroom

[0084] When the mushroom is assumed to be a hemisphere, the followingheat balance is established.

k′=M ²×(T−Ts)×Q ^(n) =a′+b′×(v1′+v2′+v3′+v4′)  (8)

[0085] In the above Equation (8), a′, b′ and n are constants, Q is aflow rate of total gas (Nm³/h/t), T is a temperature (° C.) of molteniron, and Ts is a solid line temperature (° C.) determined by thecomposition of molten iron. In this case, v1′, v2′, v3′ and v4′ can becalculated when a ratio of contribution to the creation of the mushroomis determined by an experiment according to the physical property andreaction heat of the used gas. Ts can be found according to the phasediagram. When these are put into Equation (8) and the constants aredetermined so that they can agree with the mushroom diameter obtained byan experiment. It is possible to obtain a equation of estimation of themushroom diameter when an actual device is used. According to theinvestigation made by the present inventors, the following were found.The ratio of contribution of the heating value by inner tube oxygen tothe diameter of the mushroom was only 3%, and the ratio of contributionof the sensible heat of inner tube gas to the diameter of the mushroomwas only 30%.

[0086] The ninth item of the present invention shows another method ofopening used when the tuyere is blocked. The method of observing theinside of a molten metal refining furnace comprises the steps of:supplying inert gas from the inner tube at all times; supplying a mixedgas, in which inert gas and oxidizing gas are mixed, or only supplyingoxidizing gas, from the outer tube, so as to increase the ratio ofopening in the tuyere opening period in the case where the ratio ofopening of the tuyere is lower than α(%) shown in Equation (5); andsupplying tuyere cooling gas or inert gas alone from the outer tube orsupplying mixed gas, in which tuyere cooling gas and inert gas are mixedwith each other, from the outer tube in a period except for the tuyereopening period. Concerning the action to open the tuyere, one of thefollowing actions (1) to (3) is executed, so that the temperature at theforward end of the tuyere is raised and the mushroom is melted.

[0087] (1) Oxygen gas is mixed with inert gas in the outer tube.

[0088] (2) Tuyere cooling gas in the outer tube is changed over tooxidizing gas.

[0089] (3) only oxidizing gas is supplied to the outer tube.

[0090] The reason why inert gas is supplied to the inner tube at alltimes and the opening is made by outer tube gas is described as follows.For example, when light of short wave-length, which is emitted by carbonor phosphorus by the action of the laser, is observed, the emitted lightis greatly absorbed by oxygen in the tube. Therefore, in order totransmit the emitted light without being attenuated, it is necessary tofill the inner tube with an inert gas at all times. According to theinvestigation made by the present inventors, it was found that even whenan inert gas is supplied from the inner tube at all times, the tuyerecan be opened when the composition of gas supplied from the outer tubeis controlled.

[0091] Specifically, in the same manner as that of the inventiondescribed in the above item (8), according to the relation shown in FIG.1, diameter M of the mushroom created at the forward end of the tuyereis controlled as M/r, which is made to be not more than 2. Diameter M ofthe mushroom can be estimated in such a manner that diameter M of themushroom is calculated by the heat balance of each of the followingitems (1) to (4).

[0092] (1) Cooling index (v1″) by sensible heat of outer tube gas:function of specific heat of outer tube gas

[0093] (2) Cooling index (v2″) by latent heat of outer tube gas:function of reaction heat of outer tube gas

[0094] (3) Cooling index (v3″) by sensible heat of inner tube gas:function of specific heat of inner tube gas

[0095] (4) Heat receiving index (k″) from molten iron of mushroom

[0096] When the mushroom is assumed to be a hemisphere, the followingheat balance is established.

k″=M ²×(T−Ts)×Q ^(n) =a″+b″×(v1″+v2″+v3″)  (9)

[0097] In the above Equation (9), a″, b″ and n are constants, Q is aflow rate of total gas (Nm^(3/)h/t), T is a temperature (° C.) of molteniron, and Ts is a solid line temperature (° C.) determined by thecomposition of molten iron. In this case, v1″, v2″ and v3″ can becalculated when a ratio of contribution to the creation of the mushroomis determined by an experiment according to the physical properties andthe reaction heat of the gas used. Ts can be found according to thephase diagram. When these are put into Equation (9) and the constantsare determined so that they can agree with the mushroom diameterobtained by an experiment, it is possible to obtain a formula ofestimation of the mushroom diameter when an actual device is used.According to the investigation made by the present inventors, thefollowing were found. The ratio of contribution of the heating value byouter tube oxygen to the diameter of the mushroom was 75%, and the ratioof contribution of the sensible heat of outer tube gas to the diameterof the mushroom was 100%.

[0098] The tenth item of the present invention provides a tuyere forexecuting a method of observing the inside of a molten metal refiningfurnace of the present invention. The reason why a double tube tuyerefor observing the temperature is adopted is that the composition andflow rate of gas in the inner and the outer tube are independentlycontrolled. In this double tube tuyere, the ratio of opening at theforward end of the inner tube tuyere is detected, and the flow rateand/or composition of gas in the inner and the outer tube is controlledaccording to the information obtained by the detection. In order toenable the above operation, the tuyere is composed as shown in FIG. 5.The tuyere is composed of a concentric double tube structure includingan inner tube 1 and an outer tube 2 penetrating refractories of arefining furnace. In this structure, the inner tube 1 and the outer tube2 are independent from each other. The flow rate and/or composition ofgas can be independently controlled via the inner tube gas supply pipe 9and the outer tube gas supply pipe 10 which are independently connectedwith the control unit to control the composition and flow rate of gas.In this case, the inner diameter of the tuyere for observation isprescribed to be 5 to 20 mm. In the case where the inner diameter of thetuyere for observation is smaller than 5 mm, it is impossible to createa mushroom when an opening area necessary for observation is ensured,and the life of the tuyere is shortened. In the case where the innerdiameter of the tuyere for observation is larger than 20 mm, the flowrate of gas is increased, and the cost is raised, which is noteconomical.

EXAMPLE

[0099] In the example, a top-blow oxygen converter, the capacity ofwhich was 3 ton, was used. A single tube tuyere, the diameter of whichwas 4 mm, which was arranged at the furnace bottom, was used as thetuyere for observation. (In this case, α in Formula (1) is 47.8.)Nitrogen was supplied alone from the tuyere. Alternatively, a mixed gasin which Ar and oxygen were mixed with each other was supplied. Molteniron of [C]: 4.2%, [Mn]: 0.16%, [Si]: 0.21% and [P]: 0.085% was chargedinto the furnace, and oxygen was supplied to the furnace fordecarbonization. When the supply of oxygen was started, the temperatureof molten iron was 1315° C. In this case, % means mass percent, which isthe same in the following descriptions. The composition at the time ofblowout was [C]: 0.04%, [Mn]: 0.07%, [Si]: 0.01% and [P]: 0.017%, andthe temperature was 1657° C. The measurement of temperature withradiation was executed by an image fiber through the tuyere forobservation. At the same time, laser beams were irradiated via thetuyere concerned, and light emitted from carbon was observed so as tomeasure the carbon concentration. The ratio of the opening was measuredby an image obtained in the image fiber observation. According to achange in the ratio of opening, the composition and flow rate of gaswere controlled.

Example 1

[0100] Under the condition shown on Table 1, the flow rate of Ar wascontrolled for each carbon concentration and temperature. As a result,it was possible to make an accurate measurement of temperature and ananalysis of carbon concentration all through the refining period. TABLE1 Time of Ratio of measure- Carbon Temperature Inner tube: Nm³/s openingment (%) (° C.) Ar Oxygen % 2σ   4-2.5 1350-1425 0.0035-0.0050 0 68-762.5-3.1 2.5-1.0 1425-1525 0.0036-0.0050 0 75-88 2.2-3.3 1.0-0.51525-1600 0.0027-0.0036 0 79-90 1.9-2.9  0.5-0.05 1600-16500.0024-0.0027 0 60-72 2.1-3.1

Example 2

[0101] The temperature rising rate was low at the beginning. Therefore,the tuyere was blocked at the point of time of [C]=about 0.05% andtemperature =1600° C. (as shown by (1) on Table 2). Therefore, thecomposition and flow rate of gas were controlled under the conditionshown by (2) on Table 2. As a result, the tuyere was opened again. Afterthat, it was possible to make an accurate measurement of temperature andan analysis of carbon concentration all through the refining period.TABLE 2 Ratio of Time of Carbon Temperature Inner tube: Nm³/s openingmeasurement (%) (° C.) Ar Oxygen % 2σ (1) 0.05 1600 0.0024 0 42-0 Impossibility Comparative of measurement Example (2) 0.05 1600 0.00240.00012 84-93 2.2-3.5 Present Invention

Comparative Example 1

[0102] In Comparative Example 1, operation was performed under thecondition shown on Table 3 while the flow rate of Ar was kept constantirrespective of the carbon concentration and temperature. As a result,the ratio of opening was decreased at the end of refining, and it becameimpossible to make observations. TABLE 3 Ratio of Time of CarbonTemperature Inner tube: Nm³/s opening measurement (%) (° C.) Ar Oxygen %2σ   4-2.5 1350-1425 0.0050 0 78-98 2.4-3.3 2.5-1.0 1425-1525 0.0050 068-82 2.9-3.6 1.0-0.5 1525-1600 0.0050 0 43-61 4.8-8.5  0.5-0.051600-1650 0.0050 0 39-0  Impossibility of measurement

Example 3

[0103] In Example 3, a top-blow oxygen converter, the capacity of whichwas 3 ton, was used. A double tube tuyere, the inner diameter of theinner tube tuyere of which was 10 to 15 mm and the interval between theinner and the outer tube of which was 1 mm, which was arranged at thefurnace bottom, was used as the tuyere for observation. Nitrogen and/oroxygen was supplied from the inner tube, and one of nitrogen, oxygen andLPG or not less than two of them were supplied from the outer tube.Molten iron of [C]: 4.2%, [Mn]: 0.16%, [Si]: 0.21% and [P]: 0.085% wascharged into the furnace, and oxygen was supplied to the furnace fordecarbonization. When the supply of oxygen was started, the temperatureof molten iron was 1315° C. In this case, % means mass percent. Thecomposition at the time of blowout was [C]: 0.04%, [Mn]: 0.07%, [Si]:0.01% and [P]: 0.017%, and the temperature was 1657° C. The measurementof temperature with radiation was executed by an image fiber through thetuyere for observation. At the same time, laser beams were irradiatedvia the inner tube, and light emitted from carbon was observed so as tomeasure the carbon concentration. The ratio of opening was measured byan image obtained in the image fiber observation in the inner tube.According to a change in the ratio of opening, the composition and flowrate of gas in the inner and outer tube were changed so as to controlthe size of the mushroom at the forward end of the tuyere of the innertube.

[0104] A double tube tuyere, the inner diameter of the inner tube tuyereof which was 15 mm, was used. Under the condition shown on Table 4,according to a change in the measured ratio of opening, while themushroom size was being estimated for each carbon concentration andtemperature, the flow rate of nitrogen in the outer tube wasappropriately controlled. As a result, it was possible to make anaccurate measurement of temperature (showing 2×σ on the table) and ananalysis of carbon concentration all through the refining period. Inthis connection, the flow rate of the inner tube was kept constant at avalue 1.5 times as high as the critical flow rate. In this connection, αin Formula (5) is 3.8% because the inner diameter is 15 mm. TABLE 4Ratio of Time of Carbon Temperature Inner tube: Nm³/h/t Outer tube:Nm³/h/t opening measurement (%) (° C.) Nitrogen Oxygen Nitrogen Oxygen %2σ   4-2.5 1350-1425 0.098 0 0.08-0.02 0 32-68 2.6-3.0 2.5-1.0 1425-15250.098 0  0.02-0.001 0 39-71 2.6-2.7 1.0-0.5 1525-1600 0.098 00.001-0.02  0 25-56 2.7-3.5  0.5-0.05 1600-1650 0.098 0 0.02-0.03 018-38 3.1-3.5

[0105] In this case, the critical flow rate (F: Nm³/h) was calculated bythe following formula.

F=5.5×(ρ_(g)/ρ₁)^(−5/8)×(1+H/1.48)^(3/8)×(r/1000)^(5/2)  (10)

[0106] In the above formula, ρ_(g) is gas density (kg/m³), ρ₁ is molteniron density (kg/m³), and H is a bath depth (m).

Example 4

[0107] In Example 4, the precondition was set to be the same as that ofExample 3, and a double tube tuyere, the inner diameter of the innertube tuyere of which was 10 mm, was used. Under the conditions shown onTable 5, according to the change of the ratio of opening that wasmeasured, the composition and flow rate of outer tube gas wereappropriately controlled while the mushroom size was being estimated foreach carbon concentration and temperature. As a result, it was possibleto make an accurate measurement of temperature and analysis of carbonconcentration through all the refining period. In this connection, theinner tube flow rate was set at a constant value which was 1.5 times ashigh as the critical flow rate. In Formula (5), α was 8.5% because theinner diameter was 10 mm. TABLE 5 Ratio of Time of Carbon TemperatureInner tube: Nm³/h/t Outer tube: Nm³/h/t opening measurement (%) (° C.)Nitrogen Oxygen Nitrogen Oxygen LPG % 2σ   4-2.5 1350-1425 0.036 0 0.01   0-0.0011 0 34-65 2.9-3.5 2.5-1.0 1425-1525 0.036 0 0.01 0.0011-0      0-0.0075 38-71 3.3-3.6 1.0-0.5 1525-1600 0.036 0 0.01 0 0.0075-0.015 35-68 2.8-3.6  0.5-0.05 1600-1650 0.036 0 0.01 0 0.015-0.02  28-513.3-3.9

Example 5

[0108] In Example 5, the precondition was set to be the same as that ofExample 3, and a double tube tuyere, the inner diameter of the innertube tuyere of which was 10 mm, was used. However, the temperaturerising rate was low at the beginning, and the tuyere was blocked at thepoint of time of [C]=about 2.4% and temperature=about 1400° C. (shown by(1) on Table 6). Therefore, the composition and flow rate of outer tubegas were changed under the condition of (1) or (2) shown on Table 6, andthe size of the mushroom at the forward end of the inner tube tuyere wascontrolled. As a result, the tuyere was opened again. As a result, itwas possible to make an accurate measurement of temperature and ananalysis of carbon concentration all through the refining period. TABLE6 Ratio of Time of Carbon Temperature Inner tub: Nm³/h/t Outer tube:Nm³/h/t opening measurement (%) (° C.) Nitrogen Oxygen Nitrogen Oxygen %2σ (1) 2.4 1400 0 0.01 0 0 Impossibility Comparative of measurementExample (2) 2.4 1400 0.036 0 0.01 0.0018 82-93 2.6-3.4 Present Invention(3) 2.4 1400 0.036 0.6 0.01 0 84-91 2.8-3.3 Present Invention

Comparative Example 2

[0109] In Comparative Example 2, a double tube tuyere, the innerdiameter of the inner tube tuyere of which was 15 mm, was used, andoperation was performed under the condition shown on Table 7 wherein theflow rate of nitrogen in the outer tube was kept constant irrespectiveof the carbon concentration and temperature. As a result, in the middleof refining, the ratio of opening was decreased, so that it becameimpossible to make observations. Further, at the end of refining, themushroom was melted and the tuyere for observation was damaged byfusion. TABLE 7 Ratio of Time of Carbon Temperature Inner tube: Nm³/h/tOuter tube: Nm³/h/t opening measurement (%) (° C.) Nitrogen OxygenNitrogen Oxygen % 2σ   4-2.5 1350-1425 0 0.02 0 22-0  5.2 →Impossibility of measurement 2.5-1.0 1425-1525 0.098 0 0.02 0  0-33Impossibility of measurement → 7.2 1.0-0.5 1525-1600 0.098 0 0.02 048-81 3.8-6.9  0.5-0.05 1600-1650 0.098 0 0.02 0 88-98 Fusion of tuyere

INDUSTRIAL POSSIBILITY

[0110] According to the present invention, it is possible to stablyobserve the temperature and/or composition of molten iron in a refiningfurnace by opening a tuyere for observation, at all times, according tothe state of refining.

What is claimed is:
 1. A method of observing the inside of a moltenmetal refining furnace comprising the steps of: using a single tubetuyere for observing the temperature and/or composition of molten ironin the refining furnace via a tube penetrating refractories of a furnacewall and/or furnace bottom of the molten iron refining furnace bydetecting electromagnetic waves radiated from molten metal at a forwardend of the tube under a non-contact state; and using an inert gas or anoxidizing gas alone or mixed with each other according to the openingcondition of the forward end of the tube.
 2. A method of observing theinside of a molten metal refining furnace according to claim 1, whereina mixed gas of an inert gas with an oxidizing gas or only an oxidizinggas is supplied in the case where the ratio (%) of opening of the tubeis not higher than α which is calculated by the inner diameter r (mm) ofthe tube according to Equation (1), and only inert gas is supplied inthe case where the ratio of opening is higher than α. α=765/r ²  (1) 3.A method of observing the inside of a molten metal refining furnacecomprising the steps of: using a single tube tuyere for observing thetemperature and/or composition of molten iron in the refining furnacevia a tube penetrating refractories of a furnace wall and/or furnacebottom of the molten iron refining furnace by detecting electromagneticwaves radiated from molten metal at a forward end of the tube under anon-contact state; and controlling a flow rate of an inert gas accordingto the opening condition of the forward end of the tube.
 4. A method ofobserving the inside of a molten metal refining furnace according toclaim 3, wherein a flow rate of inert gas is controlled according to thetemperature and composition of molten iron so that the ratio (%) ofopening of the single tube can be not less than α, which is calculatedfrom the inner diameter r (mm) of the tube according to Equation (1),and not more than 95%.
 5. A tuyere for observing the inside of a moltenmetal refining furnace having a single tube for observing thetemperature and/or composition of molten iron in the refining furnacevia a tube penetrating refractories of a furnace wall and/or furnacebottom of the molten iron refining furnace by detecting electromagneticwaves radiated from molten metal at a forward end of the tube under anon-contact state, the tuyere for observing the inside of a molten metalrefining furnace comprising a control function by which an inert gas oran oxidizing gas can be used alone, or mixed with each other, accordingto a state of opening of the forward end of the tuyere tube, the innerdiameter of which is 2 to 6 mm.
 6. A method of observing the inside of amolten metal refining furnace in which the temperature and/orcomposition of molten iron in the molten iron refining furnace isobserved via a tube penetrating refractories of a furnace wall and/orfurnace bottom of the molten metal refining furnace in a non-contactstate by detecting electromagnetic waves radiated from molten iron atthe forward end of the tube, the method of observing the inside of themolten metal refining furnace comprising the steps of: using a twin tubetuyere; detecting a ratio of opening of the forward end of the innertube tuyere; and controlling a size of a mushroom at the forward end oftuyere by changing gas flow rate and/or gas composition of which issupplied through the inner and the outer tube according to a change inthe ratio of opening so as to keep the ratio of opening necessary forobservation.
 7. A method of observing the inside of a molten metalrefining furnace according to claim 6, further comprising the steps of:estimating the size of the mushroom at the forward end of tuyereaccording to the temperature and composition of molten iron; andcontrolling the size of the mushroom at the forward end of tuyere bychanging the gas flow rate of and/or gas composition of LPG inert gas,an inert gas and an oxidizing gas which are supplied through the outertube according to the result of the estimation so as to keep the ratio(%) of opening of the inner tube in a range from not less than α (%),which is calculated by Equation (5), to not more than 95%. α=850/r²  (5) where r is an inner diameter (mm) of the inner tube.
 8. A methodof observing the inside of a molten metal refining furnace according toclaim 6, further comprising the steps of: supplying mixed gas, in whichan inert gas and an oxidizing gas are mixed, or only an oxidizing gas,from the inner tube so as to increase the ratio of opening in a tubeopening period in the case where the ratio of opening of the inner tubeis lower than α (%) in Equation (5); and supplying only an inert gasfrom the inner tube in a period except for the tube opening period.
 9. Amethod of observing the inside of a molten metal refining furnaceaccording to one of claims 6 to 8, further comprising the steps of:supplying an inert gas from the inner tube at all times; supplying amixed gas, in which an inert gas and an oxidizing gas are mixed witheach other, or only an oxidizing gas from the outer tube so as toincrease the ratio of opening of the inner tube in a tube opening periodin the case where the ratio of opening of the inner tube is lower than α(%) in Equation (5); and supplying tuyere cooling gas or an inert gasalone through the outer tube or supplying a mixed gas, in which tuyerecooling gas and an inert gas are mixed, from the outer tube in a periodexcept for the tube opening period.
 10. A tuyere for observing theinside of a molten metal refining furnace which is double tube tuyerefor observing the temperature and/or composition of molten iron in therefining furnace via a tube penetrating refractories of a furnace walland/or furnace bottom of the molten iron refining furnace by detectingelectromagnetic waves radiated from molten metal at a forward end of thetube under a non-contact state, the tuyere for observing the inside of amolten metal refining furnace comprising: a piping structure; and acontrol system capable of independently controlling a gas flow rateand/or gas composition which is supplied through each of the inner andthe outer tube.
 11. A tuyere for observing the inside of a molten metalrefining furnace according to claim 10, wherein inner diameter r of theinner tube is 5 to 20 mm.